The party guests gathered excitedly around the serving table, all eyes on the cake with white icing and pastel polka dots.

Expectant parents Minda and Jacob Campbell were about to discover whether to prepare for a son's or daughter's arrival in April.

Friends and family watched with anticipation as the Campbells each slid a knife into the layer cake and lifted out a slice of pink cake.

"It's a girl!" they said, laughing with delight as their guests cheered.

"Our friends had one of these parties a month ago, and it was so much fun," said Mrs. Campbell. "We were all standing around the cake waiting for them to put the knife in. The excitement got so built up waiting for that moment."

The Campbells are among a growing number of couples across the country choosing to announce the sex of their developing bundles of joy at a baby-gender reveal party.

The "reveal cake" is decorated in a gender-neutral design on its exterior, but the cake layer has been custom-tinted with food coloring, either blue for a boy or pink for a girl.

Jennifer Downey, of Jackson Bakery in Brainerd, which created the Campbells' reveal cake, said it's a fad she believes started in 2009 when Josh and Anna Duggar -- Josh is the eldest child of the Duggar family of TLC's "19 Kids and Counting" series -- sliced into a cake on NBC's "Today" show to discover the sex of the Duggars' first grandchild.

"Shortly after that, we got our first order," said Downey. "We just didn't realize at the time that it would be the start of such a fad."

When the expectant couple's obstetrician or sonographer discerns the baby's sex during an ultrasound, the couple can opt not to be told. The parents-to-be look away as the sonographer prints out an image of the fetus, writes down the baby's sex and seals it in an envelope.

The future parents deliver the envelope to the bakery of their choice, where the baker will open it before baking their custom cake with either blue- or pink-tinted batter.

"Sometimes doctors' offices call us directly. Sometimes we get photos double-sealed in a second envelope, to make sure the parents aren't tempted to look," said Downey, laughing.

"It's weird that we know for a couple of days what they're having when they don't even know yet," she said. "It's all about the excitement and buildup."

Mark Houghton, owner of Piece a Cake Bakery in Ooltewah, said it's not unusual for the excited couple to want to share their big news at a party the night of their ultrasound.

To accommodate that quick turnaround, he bakes two cakes for the couple, one with pink layers, the other with blue. When the mom-to-be arrives with her ultrasound results, he reads it, then marks the box containing the correct cake.

It wasn't that many years ago that parents had to wait until the birth to learn their baby's sex. But technological advances have made it possible for ultrasounds to determine gender as early as 16 weeks.

"Usually between 18 to 20 weeks, we'll do an ultrasound to determine the baby's sex," said Nancy Miller, a registered diagnostic medical sonographer with The Galen Group.

The sonographer said 95 percent of all expectant parents she sees want to know their child's sex in advance in order to prepare nurseries and buy gender-appropriate clothing and baby gear.

Miller said she first heard of baby reveal parties about three months ago when she got her first request for a sealed ultrasound.

"I've gotten a lot more requests in recent weeks. The parties have really gotten popular," she said.

Kimberly Beattie, owner of Couture Cakes & Confections in Hixson, said she got her first order for a baby reveal cake about two years ago.

"But we ship nationwide, so we've been seeing this fad about two years. The parties have become popular locally since then."

Beattie said it's not uncommon to have relatives, even parents, try to wheedle results out of them ahead of time.

"Do we tell? Never!" said the baker.

Jeremy and Leslie Lamb used their last name as the inspiration for a lamb-shaped reveal cake at their family party but pulled a color switch on their guests.

"Instead of pink or blue inside, we used yellow for a girl and green for a boy," said Mrs. Lamb.

She explained her husband had wanted to know their baby's sex before the rest of the family, so this way the two of them had that bonding moment at the party when the cake was cut because only they knew the color code.

"When we cut into the cake, our family had no idea what to think," said Mrs. Lamb. "They were like 'Green! ... Yay?' The expressions on their faces in the video are hysterical."